Curve modifier on an already curved model?
As soon as your curve is not flat, it will deform the shape! The curve will deform the shape as much as the curve is itself deformed compared to a flat curve.

So, "do I have to model the mesh in a straight line", I would say yes.

Note that you need to put the origin of the curve at its beginning and make sure that its normals go in the good direction.



Does it answer your question?